Area detonation



Filed Oct, 13, 1958 flrraewers 3,024,727 Patented Mar. 13, 1962 3,024,727 AREA DETONATION Joseph R. Hradcl, Mount Pleasant, Mich., assignor, by mesne assignments, to The Dow Chemical Company, Midland, Mich, a corporation of Delaware Filed Oct. 13, 1958, Ser. No. 766,873 4 Claims. Cl. 102-23) The present invention comprises a process for detonating a relatively large area by use of a patterning of shot holes having a spacing of about 1 hole per acre. The process is most amenable to open pit mining operations where an entire bench or face is to be broken in relatively heavy ground. This application is filed concurrently with my copending application entitled, Explosive Compound and Process for Use, Serial Number 763,908, filed September 29, 1958, now abandoned, constituting a continuation-impart thereof and constitutes a continuation-in-part of my previously filed application, Serial Number 444,383, filed on July 19, 1954, now issued as U.S. Patent No. 2,867,172, entitled Detonation of Unprimed Base Charges and my application Serial Number 750,481, filed July 23, 1958, and entitled Axial Detonation and Device Therefore, now abandoned. In the earliest filed application I taught means and process making possible the detonation of base charges forced into a geologic formation by use of a Munroe jet or shaped charge. In that application the explosive material was slurried with water and/or oil with or without acid for well treating and the like. It was therein demonstrated that the shaped charge was an effective detonating means through intervening strata, casing, rock, and fluid.

In the application filed on July 23, 1958 (Serial Number 750,481), now abandoned, I expressed the discovery that a single jet, properly oriented could and did accomplish substantially complete detonation of a vertical and columnar mass of explosive material.

Analysis accompanying the work encompassed by the expression in the above indicated applications shows that the Munroe jet or shaped charge, when employed to detonate relatively insensitive base charges does not conform to conventionally accepted priming practices in that: (a) normal weight ratios between primer charge and base charge are radically changed in view of the fact that a one and three quarter ounce shaped charge has initiated up to 25,000 pounds of ammonium nitrate base load (b) the conventional geometric distribution of prime load within a base load mass is obsolete when the shaped charge is employed for initiation. Thus, the large scale firing of a sprung hole is feasible with insensitive loads which results in substantial shooting economy as a consequence of lower powder costs, lower drilling costs, use of a single detonator per shot, and all under safer conditions than heretofore experienced under prior shooting procedures.

Having made these disclosures, I then discovered a new explosive composition comprising a combination of ammonium nitrate, a metal thermal carrier, and water as a sensitizing agent. The dry materials were simply dumped into the borehole and the water was sprayed in simultaneously with the dumping operation. The whole was then aged in situ. In my test work on this material working in taconite deposits in Calumet, Minnesota, I discovered that the strength factor of ammonium nitrate could thus be materially raised. The Munroe type jet or shaped charge was used as an effective detonator. Using a single such shaped charge oriented upwardly from the bottom of a borehole and directed upwardly along the borehole axis, and using a five hundred pound load of the described explosive material, the resulting shot brought down a conservative 10,000 tons of rock. Skeptics were amazed at the consequence of the shot which pointed to an entire new conception of open pit shooting. The importance of this discovery will be appreciated when it is considered that previous best results were obtained using ammonium nitrate sensitized with oil and primed with ten percent TNT. The best obtained breakages have been about 4 tons per pound of explosive. My procedures raised that ratio of breakage to about 20 tons per pound of explosive. More important still is the fact that my procedure makes possible the use of a spacing pattern for holes using about 1 hole per acre as against the previously required 225 holes per acre and a total base load of about 46,000 pounds of explosive, based on .65 pound per cubic yard loading presently being used. I have discovered that by springing each hole with about 200 pounds of my explosive material, then reloading the cooled sprung hole with about 10,000 pounds of my explosive material that the entire acre can be adequately broken. The depth of the holes is about forty five feet on the average. This constitutes a remarkable operational saving in hard rock mining.

Hence the present invention has as its principal object the teaching of a highly efficient area blasting process wherein a single borehole per acre, when shot, accomplishes adequate breakage in hard rock (300-330 pounds per cubic foot).

Another object is to accomplish the above procedural result while employing a single shaped charge per shot and using a relatively inexpensive explosive material.

Still another object of the present invention is to accomplish approach to a breaking ratio in hard rock of about 20 tons per pound of explosive material.

Other objects will become increasingly apparent to those skilled in the art as the description proceeds.

In the drawing:

FIGURE 1 is a schematic plan view of an open pit mining operation. Each of the contour lines indicates a particular bench level. The upper bench is cross hatched to indicate the area of ground to be broken and the selected spacing between boreholes required to shoot the illustrated 4 acre area is indicated.

FIGURE 2 is a schematic cross sectional elevation taken on the line II-II in FIGURE 1 and showing a hole loaded without tamping in accord with the present invention with the shaped charge detonator in position for firing with the principal axis of the jet oriented upwardly along the axis of the borehole.

FIGURE 3 illustrates the hole shown in FIGURE 1 enlarged by the springing shot and demonstrates the amount of cratering accomplished by the loading shown in FIGURE 2. The crater is shown reloaded for the final break shot with tamping in position and with the shaped charge detonator in position for final firing. The cross hatched area in FIGURE 3 shows the schematic amount of final break.

Referring more particularly to the drawings an open pit mine II is shown in FIGURE 1. The mine II comprises a plurality of benches 13, 14, and 15. Each of the benches is maintained at a substantially constant level and during the mining operations a bench is moved back by blasting and the debris falls upon the surface of the next lower adjacent bench. To accomplish this it has been customary to establish a pattern of vertical holes back from the face of a given bench. These holes are then loaded and the bench is fired to break out the rock. in the FIGURE 1, the uppermost bench 15 is shown prepared for shooting approximately 4 acres (shaded area 16) of rock using only 4 holes to accomplish the break and spacing the holes 17, 18, 19, and 20 back about 104 feet from the bench face 21 and having a spacing interval of about 208 feet between hole centers.

In FIGURE 2, the borehole 20 is illustrated as it is loaded for springing. It will be appreciated that holes 17, 18, and 19 are similarly loaded without tamp with about 200 pounds of explosive material 22 comprising a mixture of granulated metal, preferably magnesium aluminum alloy, and ammonium nitrate. Water is uniformly added to the material and the material 22 is allowed to stabilize to ambient borehole temperature so that recrystallization between adjacent and ammonium nitrate particles occurs. The water is dispersed throughout the dry ingredients at the head of the borehole as by a fine spray nozzle playing a water stream on the dry material as it is poured into the hole. This material 2 2 is poured into the borehole 20 on top of a shaped charge 23, at the end of shooting lines 24. The shaped charge 23 is oriented at the bottom of the hole 20 so that its principal axis is directed upwardly through the explosive material 22, substantially along the principal axis of the borehole 20. No tamp is required since it is desirable to achieve maximum springing and clean out of debris. The borehole 20 is fired by detonation of the shaped charge 23. The amount of explosive material 22 used in a 6 inch diameter hole at a depth of approximately 45 feet is about 200 pounds. In this initial firing the hole 20 is sprung creating strains in the rock which strains emanate from hole 20. The crater pocket 25, thus formed, is shown in phantom line in FIGURE 2 and in FIGURE 3 is shown as reloaded for the final breakshot.

The hole 20 and its crater 25 are allowed to cool from the shot and the h6le 20 and crater 25 are reloaded with the previously described explosive material 22. AS before, a shaped charge 23' is first suspended from the top of the borehole 20 by its shooting lines 24- and the explosive material 22 is poured into the crater 25 so as to substantially fill the crater 25 allowing room for adequate tamping 26. This final break load is about 25 times greater than the amount of explosive material 22 used in springing the hole. Sensitizing water is added as in the springing shot in an amount between about 1.14 and 5.68 percent of the total weight of the explosive material 22 and time is allowed for curing to ambient borehole temperature.

The single shaped charge 23 accomplishes complete detonation of the wetted explosive material 22 in the crater 25 and the entire face is broken free and heaved onto the next adjacent lower bench. The break thus accomplished in taconite material reduced the debris size to well within the size for handling by moderate sized powered shovels. No extensive plugging was required to prepare the resulting rubble for removal.

Each of the boreholes 17, 18, 19, and 20 were. prepared similarly in a single line spaced back from the bench face and centered in approximate one acre areas. In the firing of the face all of the boreholes were fired simultaneously with excellent results in both springing of the holes and in the final break. In some instances sequential firing in timed sequence was used with equally good results.

Overall economies realized from this new procedure included minimizing of the number of boreholes, simplification of open pit blasting procedures, cheap powder, and very good safety practice for loading and detonating.

Having thus described my invention certain other modifications will become apparent to those skilled in the art and such modifications are intended to be included herein limited only by the scope of the hereinafter appended claims.

I claim:

1. In a process for area detonation the steps which include: springing an untamped borehole with a relatively light charge of wetted explosive material; breaking with a second heavier tamped and wetted charge of about 40 to 50 times the size of said springing charge; and accomplishing detonation of each charge by means of a shaped initiating charge, said shaped charge being placed at the bottom of said borehole and upwardly directed.

2. In a process for achieving rock breakage by blasting the steps which include: preparing an untamped hole with a first light charge of wetted explosive material including granular magnesium alloy inserted in the hole over an upwardly directed shaped charge, said shaped charge being oriented at the bottom of said hole so that its principal axis is directed upwardly through the explosive material substantially along the principal axis of the borehole; firing said shaped charge along the axis of said borehole thereby springing said borehole; tamping in a second charge of identical explosive material into the thus cratered borehole over a second upwardly directed shaped charge positioned in the same manner as the first shaped charge; and firing said second shaped charge along the axis of said borehole and through said explosive material.

3. In a process for area blasting the steps which include: locating a line of boreholes spaced on approximate acre centers; loading each of said boreholes with a mixture of ammonium nitrate and granular metal; adding water to said mixture; allowing said mixture to recrystallize; detonating said mixture by means of a single shaped charge directed axially through said mixture along the axis of said borehole said shaped charge being positioned at the bottom of said borehole; allowing said borehole to cool from said detonation and reloading said borehole with a tamped charge as above described but using an amount of charge about 50 times as large as the initial springing charge; and detonating the hole for final break with a single shaped charge said shaped charge being positioned at the bottom of said borehole and having its principal axis directed through said breaking charge and substantially along the axis of said borehole.

4. In a process for area blasting: locatinga plurality of boreholes on approximate acre centers along an adjacent bench; detonating a first light load of wetted explosive material in said boreholes; detonating a second heavier load of wetted explosive material in the thus cratered boreholes using a single shaped charge initiator placed at the bottom of each of said boreholes in contact with said explosive material and each of said shaped charges being oriented so that the principal axis of said shaped charges are substantially coaxial with the axis of each of said boreholes.

References Cited in the file of this patent UNITED STATES PATENTS 351,959 Bourne Nov. 2, 1886 371,298 Bourne Oct. 11, 1887 2,745,346 Aitchison et al. May 15, 1956 2,836,484 Streng et al. May 27, 1958 2,867,172 Hradel Jan. 6, 1959 

